The invention relates to foam having an apparent density of less than 100 kg/m3 prepared with water as the predominant blowing agent and to uses thereof. Further the invention relates to foams having a large proportion of open cells ( greater than 80%).
Rigid polyurethane foams are generally distinguished by their low thermal conductivity. Foams having closed cells are used for the heat insulation of hot-water cylinders, buildings, refrigerators and cooling cells, doors, pipes and tanks. Often rigid foams are also used in direct conjunction with other materials. Closed-cell polyurethane foams are nowadays mostly produced using physical blowing agents such as chlorofluorohydrocarbons, fluorohydrocarbons, hydrocarbons and chlorofluorocarbons. Besides their considerable advantages in improving heat insulation, such materials often have disadvantages with environmental compatibility or safety in the workplace due to their effect on the ozone layer or due to their combustibility. Their use is therefore limited by international agreements or is associated with, in some cases, considerable costs as a result of the necessary safety measures.
If physical blowing agents are dispensed with and water is used on its own as a chemical blowing agent, good heat-insulating foams can likewise be produced. Due to the uneven rates of diffusion of carbon dioxide out of the cells of the foam and of air into those cells, a sub-atmospheric pressure will result in the cells when the foam contains predominantly closed cells. The mechanical load associated therewith can make it necessary for closed-cell rigid foams blown solely with water to have a higher strength and therefore a higher weight per unit volume than foams produced using physical blowing agents. Foams of good dimensional stability and having the same, or lower, weights per unit volume as those customarily achieved using physical blowing agents can be obtained if a high content of open cells is established in the foam. The thermal conductivity is then slightly higher, but is still adequate for many of the above-mentioned applications.
There are various possible methods of adjusting the content of open cells. First of all, polyol formulations in the form of a suspension of fine particles or in the form of emulsions are suitable. Such particles can be fluorinated polymers, as disclosed in U.S. Pat. Nos. 5,250,579, 5,856,678, and 5,721,284, salts of saturated carboxylic acids (including fatty acids) having divalent or lithium cations, see for example U.S. Pat. Nos. 5,262,447 and 5,457,138, WO 96/37533, EP-A 547 515, and EP-A 581 191, or thermoplastic polymers such as SAN or polyethylene, as disclosed in U.S. Pat. No. 5,457,138, WO 99/60045, and EP-A 1 108 514, which are added to the polyol component. According to U.S. Pat. Nos. 5,519,068 and 5,318,997, the addition of from 5 to 55% of a long-chain polyether having an OH number  less than 60 and an EO content  less than 5% is equally as suitable as the addition of extremely short-chain monofunctional alcohols, see also U.S. Pat. No. 5,889,067. Typical cell-opening additives are also special silicones or polyolefin waxes, as described in U.S. Pat. Nos. 5,852,065 and 5,614,566.
Typical disadvantages of such approaches are the separation of water-containing polyol formulations during storage, and the sedimentation of fine particles. For example, it is known that PTFE-containing polyols must be stirred before processing. SAN-containing polyol formulations are phase-stable only in the absence of the blowing agent water. At high water contents, such as are necessary in order to obtain foams of low apparent density, slow hydrolysis with mediation of the amines contained in the polyol can occur over the period of storage in the case of formulations containing polyester polyols.
It has now been found that those problems can be solved by suspending, emulsifying or dissolving polymers in an isocyanate and reacting the isocyanate-containing component so obtained with a water-containing polyol formulation typical for rigid polyurethane foams in a NCO/(OH+NH) ratio of from 0.5 to 3.5.